Period # Notes: Electronic Structure of Atoms The logicl plce (for civil engineers) to begin in describing mterils is t the tomic scle. The bsic elements of the tom re the proton, the neutron, nd the electron: prticle proton neuton electron mss (grms).673 x -4.675 x -4 9.9 x -9 electricl chrge (Coulombs).6 x -9 -.6 x -9 n n n3 The protons nd neutrons of toms occupy the nucleus which is the tight inner core region hving dimeter of order -4 m (or -5 nm). Electrons swirl round the nucleus in orbits hving rdii rnging from bout.5nm to few nm. The tomic rdius is typicl mesure of the size of n tom, nd usully corresponds to the rdius of the outermost orbitl tht is t lest prtilly full. The field of prticle physics nd quntum mechnics is devoted to understnding electron orbitls, spins, nd energy levels. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow
In very generl terms, the electrons of toms tend to fill the lowest (innermost) quntum shells first, nd the higher shells re occupied s the inner shells fill up. The energy levels of electrons in the lowest quntum shells re lower thn those in higher shells. In this wy, mterils re like most other physicl systems in tht they tend to reside t sttes of minimum potentil energy. For given mteril system, there usully exists more thn one stte of minimum potentil energy. Electron quntum sttes: (through n4) Primry Quntum # (n) 3 4 Subshell or orbitls s s p s p d s p d f # electrons per subshell 6 6 6 4 # electrons per shell (n ) 8 8 3 Generl order in which electrons fill orbitls 7s 7p 7d 6s 6p 6d 6f 5s 5p 5d 5f 4s 4p 4d 4f 3s 3p 3d s p s 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow
The energy difference for n electron moving from n inner quntum shell to n djcent outer quntum shell cn be quntified. For exmple, energy eqution of Niels Bohr for the electrons of the hydrogen tom is given below in Eq. () where E is the energy level in electron-volts (ev) nd n is n integer clled the primry quntum number. 3.6 E ev n () The energy level for n electron in quntum shell (n) is: E -3.6eV; The energy level for n electron in quntum shell (n) is: E -3.4eV; The energy level for n electron in quntum shell 3 (n3) is: E 3 -.5eV; Hydrogen tom: ) energy gined s electron moves from n to n3; b) energy lost s electron moves from n3 to n. n n n3 ΔE photon (gined) ) b) n n n3 ΔE photon (lost) 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 3
Looking t mterils from quntum mechnics point of view, the tomic energy tends to be minimized when the outermost quntum shells re either completely full or completely empty. The fcts tht () mterils tend to reside t sttes of minimum potentil energy nd () tomic energy is minimized with outermost quntum shells either completely empty or full helps to explin why some commonly found chemicl compounds occur. Electro-positivity nd Electro-negtivity Electro-positive elements: metllic in nture; tend to give up electrons to form positively chrged ions (ctions); typiclly hve 3 or fewer electrons in outermost shells. Electro-negtive elements: non-metllic in nture; tend to ccept electrons to form negtively chrged ions (nions); typiclly hve 4 or more electrons in outermost shells. Electronegtivity: mesures degree to which n tom ttrcts electrons to itself. Vlues rnge from to 4. The most electropositive elements (lest electronegtive) re the lkli metls which come from the left side of the periodic tble: A few common exmples: Potssium (.9); Sodium (.); Lithium (.); Clcium (.) Among the most electronegtive elements, which generlly come from the right side of the periodic tble re: Fluorine (4.); Oxygen (3.5); Nitrogen (3.); Chlorine (.9) 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 4
Types of Atomic nd Moleculr Bonding:. Primry bonding types:. Ionic (electron trnsfer from electro-positive elements to electro-negtive elements; coulombic forces of ttrction between ions; bonds formed re non-directionl & strong; b. Covlent (electron shring); bonds formed cn be strong; often directionl; c. Metllic bonding; electron shring by generlized dontion; bonds formed re strong non-directionl.. Secondry bonding types:. Permnent dipole bonds; b. Fluctuting dipole bones; 3. Generl comment: Mny of the physicl, electricl, nd therml properties of civil engineering mterils re dependent upon the type of bonding tht occurs between toms of compounds. A generl mesure of the strength of the bonds forming chemicl compound is the melting temperture. Essentilly this corresponds to the energy level required to brek the primry bonds of the mteril. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 5
. Ionic Bonding: Tends to occur mong elements with lrge differences in electronegtivities (i.e. those coming from the left nd right sides of the periodic tble); Common exmple: Sodium chloride: N: -8- (s s p 6 3s ) --» N -8 Cl: -8-7 (s s p 6 3s 3p 5 ) --» Cl - -8-8 Melting temperture of NCl: T m 8 C Another exmple: Mgnesium oxide: Mg: -8- (s s p 6 3s ) --» Mg -8 O: -6 (s s p 4 ) --» O -- -8 Melting temperture of MgO: T m 64 C Yet nother exmple: Zirconium crbide: Zr: -8-8- (s s p 6 3s 3p 6 4s ) --» Zr -8-8 C: -4 (s s p ) --» C ---- -8 Melting temperture of Zr C: T m 35 C Observe: the higher the degree of ioniztion in the compounds, the stronger the compound. Also, when n tom gives up its outermost electrons to become ction, its rdius decreses. Similrly, when n tom ttrcts electrons to become n nion, its rdius increses. The structure of most compounds with ionic bonding is crystlline (3-dimension). 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 6
) Attrction/Repulsion Reltions in Ions (Condon-Morse) ttrction force repulsion force net force F net intertomic spcing F ttrctive ZZe 4πε where : Z,Z is # of electrons involved in ion formtion; F F e electron chrge; interionic spcing; ε 8.85 nb repulsion n ZZe 4πε net n /( N m The equilibrium tomic spcing t which F net is. C ) where : n nd b re constnts with 7 < n < 9; nb is intertomic spcing. Notes: () As the spcing gets smller, the repulsion force domintes; () The bonds between two ions re broken only in tension, not compression. (3) This helps to explin why mterils generlly fil in tension rther thn compression. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 7
b) Energy Considertions: ttrction force repulsion force net force intertomic spcing () The equilibrium tomic spcing is lso the spcing t which the energy in the ttrction/repulsion forces is miniml. Consider now two ions widely seprted so tht there is no force of ttrction of repulsion between them. As the ions re moved closer, the forces of ttrction/repulsion kick in. The energy ssocited with the ionic forces is: ( ) F ZZe 4πε ( ) net b ( ') d' n Is often referred to s the surfce energy of compound. This represents the energy difference between n tom t equilibrium within the lttice of the compound, nd n tom t the free surfce of the mteril infinitely seprted from its nerest neighbor. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 8
53:86 Civil Engineering Mterils, Period # 9 C.C. Swn, The niversity of Iow c) Elstic Behvior of Mterils: The energy in the bonds between toms cn be expressed s function of spcing s follows: Since the energy in the bonds is minimum t the equilibrium spcing, the first order term in the Tylor series expnsion vnishes. So, in effect the energy in the bonds is qudrtic function of for smll perturbtions in tomic spcing. This is nlogous to the behvior of liner spring for which: where K is the spring stiffness, nd x is its devition from n equilibrium position. The force in the spring is of course: In the lttice of mteril, the force on n tom due to smll chnge in spcing cn be expressed: In this expression (- ) is essentilly the strin in the lttice, nd the second derivtive tken t is n elstic modulus. ( ) ( ) ( ) ) ( ) ( 4 e Z Z ) ( b n πε x K x K x F * ( ) * ) ( F
. Covlent Bonding: (shring of electrons between two toms of similr electro-negtivities) ) Molecules obtined from covlent bonding n be: Ditomic (ech tom hs just one neighbor) String-like (ech tom hs two neighbors) Plte-like (ech tom hs 3 neighbors) 3-dimensionl nd crystlline (ech tom hs 4 neighbors) weker stronger The type of compound tht will be formed cn sometimes be predicted with the 8-N rule of covlent bonding: n tom cn hve t most 8-N nerest neighbors in the molecule, where N is the number of vlence electrons in the tom. b) Exmples of covlently bonded ditomic molecules: The chlorine tom (N7) hs the following electron structure: Cl -8-7 [s s p 6 3s 3p 5 ]. In the third quntum shell, there re seven electrons. The shell would be full if there were just one more electron. This cn be chieved by electron-shring s follows: cl cl cl cl 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow
Oxygen (N6) lso forms ditomic molecule; O -6 [s s p 4 ]. In the second quntum shell, there re six electrons. The shell would be full if there were just two more electrons. This is chieved by shring of two electrons mongst two oxygen toms. c) String-like molecules: Sulfur -8-6 with N6 forms string-like molecules with ech tom hving two nerest neighbors. String-like molecules re common in polymers, which generlly speking re not high-stiffness, high-strength mterils. d) 3-d covlent molecules: Crbon [C -4] with N4 forms 3-d molecules hving four nerest neighbors. Covlently bonded crbon molecules comprise dimonds which re mong the stiffest, strongest mterils in existence. 3. Metllic bonding: ) Elements tht feture metllic bonding typiclly hve -3 vlence electrons in their outermost quntum shells. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow
b) The vlence electrons seprte from the tom (ction) nd form n electron cloud tht swirls within the lttice formed by the ctions. c) The vlence electrons re not closely ssocited with ny one specific ction. d) In this sense, the shring of electrons mongst the ctions is be generlized dontion. e) The electron cloud in metls helps to explin these mterils typiclly hve high therml nd electricl conductivities. f) The lttice structure of ctions in metls is very three-dimensionl. Hence metls re not comprised of string-like or plte-like molecules. For this reson, they tend to be reltively strong. g) Alloying in metls: When the ctions of metls form themselves in highly structured lttices, it is possible for ctions of different metls to tke the plce of the predominnt metl ctions. This is generlly clled substitutionl lloying nd it occurs when the foreign ction hs n tomic rdius similr in size to tht of the predominnt metl nd similr electronegtivity. When the foreign metl ction hs n tomic rdius much smller thn tht of the predominnt ctions, then interstitil lloying my occur. substitutionl lloying interstitil lloying 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow
4. Vn der Wls bonds The three prior types of bonding (ionic, covlent, metllic) re the primry ones in tht they form stble compounds in which the net chrge blnces re stisfied. Often, the molecules of the stble compounds formed by primry bonding mechnisms, while hving no net chrge, will hve some polrity. Tht is, molecule might hve positive side with reltively few electrons, nd negtive side with reltively mny electrons. The inter-ttrction between the opposite poles of different molecules is clled vn der Wls bonding. sully, the strength of such bonds is very smll reltive to the strengths of the primry bonds. A specil cse of vn der Wls bonding occurs with wter molecules (H O). In the wter molecule, the two hydrogen toms tend to congregte on one end (positive end) nd the oxygen molecule is on the other (forming the negtive pole). In liquid wter, the strength of the secondry ttrction/repulsion forces between polr wter molecules cn be s high s / th to / th s the primry bond forces. H H O - 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 3
Hydrogen bonding ssocited with polr wter molecules plys vitlly importnt role in the chemicl rection tht occurs when wter rects with portlnd cement. Polr wter molecules bond together different silictes nd lumintes in portlnd cement to form the product hydrted cement pste. 53:86 Civil Engineering Mterils, Period # C.C. Swn, The niversity of Iow 4